When an object to be observed is observed or an object to be processed is processed, light output from a light source such as a laser light source is condensed and irradiated onto the object to be observed or the object to be processed through a condensing irradiation optical system including lenses, etc. In such a case where light is condensed, it is known that a beam waist diameter as a standard of the size of a condensed light diameter can be reduced only to approximately half of the light wavelength. This is called the diffraction limit. However, this diffraction limit is in the case of Gaussian mode (or fundamental mode) light. On the other hand, the presence of high-order mode light with a spatial structure finer than the diffraction limit is known.
As light beams with this property, a Bessel beam and Laguerre-Gaussian mode light (hereinafter, referred to as “LG mode light”) are known. By using such a light beam, the energy of the light can be effectively concentrated at a minute region not more than the diffraction limit. For example, in Non-Patent Documents 1 to 6, light source devices which output LG mode light are described.
In Non-Patent Document 7, a technique for forming a minute spot by condensing linearly polarized light with a uniform distribution by using a three-ring (3R) filter is described. Further, in Non-Patent Documents 8 to 10, a technique for forming a minute spot by condensing a radially polarized laser beam by a high-NA lens is described.    Non-Patent Document 1: J. Arlt, et al., Journal of Modern Optics, Vol. 45, No. 6, pp. 1231-1237 (1998).    Non-Patent Document 2: D. G Grier, Nature, Vol. 424, pp. 810-816 (2003).    Non-Patent Document 3: M. W. Beijersbergen, et al., Optics Communications, Vol. 112, pp. 321-327 (1994).    Non-Patent Document 4: K. Sueda, et al., Optics Express, Vol. 12, No. 15, pp. 3548-3553 (2004).    Non-Patent Document 5: N. R. Heckenberg, et al., Optics Letters, Vol. 17, No. 3, pp. 221-223 (1992).    Non-Patent Document 6: N. R. Heckenberg, et al., Optical and Quantum Electronics, Vol. 24, No. 9, pp. S951-S962 (1992).    Non-Patent Document 7: M. Martinez-Corral, et al., Appl. Phys. Lett. Vol. 85, No. 19, pp. 4319-4321 (2004).    Non-Patent Document 8: R. Dorn, et al., Phys. Rev. Lett. Vol. 91, No. 23, p. 233901 (2003).    Non-Patent Document 9: S. Quabis, et al., Opt. Commun. Vol. 179, pp. 1-7 (2000).    Non-Patent Document 10: Y. Kozawa, et al., J. Opt. Soc. Am. A, Vol. 24, No. 6, pp. 1793-1798 (2007).